The present invention relates to an automated method and apparatus for bending metal sheets for large electrical enclosures such as enclosures for electrical transformer tanks. In electrical power distribution systems, distribution transformers are used to step down voltage between the high voltage power line and the user. Transformers are typically mounted above ground on a junction pole, or at ground level on a pad or platform. The increased use of underground power distribution systems has resulted in a corresponding increase in the number of pad-mounted transformers. The transformer includes a tank, which contains the core and coil assembly immersed in oil, and a cabinet, which includes a top hinged door and a bolted in place sill. Connections for incorporating the transformer assembly into the power distribution system extend through one wall of the tank and are enclosed by the cabinet. In order to provide utility personnel the necessary access to the transformer connections, the cabinet must also include a door. The cabinet door is pivotally attached to the tank along the top edge of the front plate of the tank. The cabinet also includes a low sill extending forward from the transformer tank, upon which the cabinet door rests when closed.
Transformer enclosures are generally fabricated from two to four metal sheets. The sheet material for these tanks are generally less than 10-gauge carbon steel or 10-gauge stainless steel. However, other kinds and gauges of metals may be used depending upon customer requirements. It is important that the method of fabrication for these enclosures be flexible enough to accommodate the broad range of enclosures that are fabricated in the transformer assembly line. The geometry of these enclosures is typically a three-dimensional box, i.e., cube-shaped or rectangular parallelepiped-shaped, but can also be extended to other shapes such as cylinders. The dimensions of the sheets that comprise these enclosures vary depending upon customer requirements. For example, a rectangular parallelepiped-shaped transformer enclosure generally comprises a front panel, a tank wrapper, a door wrapper, and a door top. Typical metal sheet size ranges, from smallest sheet size to largest sheet size, are 39".times.32" to 55".times.36",58".times.25" to 90".times.32",64".times.19" to 80".times.27",32".times.17" to 36".times.23-, for the front panel, tank wrapper, door wrapper, and door top, respectively.
The metal sheets are bent at angles, usually 90.+-.1.degree. but may include other angle measurements, to form the electrical enclosures. The bending machine is used to bend right angle bends, hem bends, and special bends for the door sheets. Bending of metal sheets are typically accomplished in bending machines such as press brake machines. Press brakes are also sometimes referred to as bending brakes, bending presses or pan brakes. Typically, a press brake is a hydraulic, mechanical, or pneumatic press which has a metal die and a metal punch known as a press brake tool which are shaped to form a particular bend or curve in the sheet metal when the die and tool are pressed together with the sheet metal in-between.
The present method to bend the enclosure requires an operator to feed the individual metal sheets into the bending machine. The metal sheets that are loaded into the machine may weigh approximately 80 to 100 pounds and are large and cumbersome to handle. It is understood that other weights and sheet sizes may also be used. Because of these difficulties in handling, this method increases the likelihood of occupational injuries.
Automation of the bending process is difficult. Each enclosure assembled is customized based upon varying customer requirements. The transformer tank assembly line, unlike traditional assembly lines, builds a variety of custom-sized pad-mounted enclosures rather than one particular size. The operator must ensure that the sheet is the correct sheet for the particular assembly being built. Lastly, and more importantly, alignment of the sheet into the bending machine is critical. The operator must ensure that the metal sheet is aligned correctly into the bending machine to ensure that the bend is placed at the correct location on sheet. A slight offset in inserting the sheet may result in scrapping or re-working of the bent sheet. For example, a 1/4.degree. rotational offset in the insertion of a 10' long sheet results in an offset of approximately 0.5" from the correct bend line.
Automated bending of properly aligned sheets into a bending machine can be accomplished using a bending machine such as a press brake. The press brake machine comprises at least two press brake fingers that act independently with respect to one another to determine whether the sheet has been received by the respective finger. The fingers are electrically coupled (via the sheet itself, for example) to a processor that communicates with a robot. As the robot inserts the metal sheet into the press brake, it adjusts the alignment of the sheet until the sheet is received by both press brake fingers. An electrical connection is formed when the metal sheet is received by the fingers. When electrical connections are formed with both fingers, the processor tells the robot that the sheet is aligned and bending of the sheet can begin. Thus, each finger acts as a sensing device to determine whether the sheet is completely inserted and properly aligned in the press brake.
In the process of inserting the metal sheet into the bending machine, however, the metal sheet removes or shaves away the insulation layer on the support member. The insulation shavings interfere with or prevent the electrical connection that is formed between the metal sheet and the conductive back plane. In addition, if the insulation is completely shaved away, thereby exposing the conductive layer underneath the layer, there may be a possibility of false alignment readings.